Print processes that employ a print head for printing swaths comprising a number of image lines in a direction perpendicular to a transport direction of an image receiving member, such as, for example, a paper sheet or paper supplied from a roll, rely on an accurate transport of the image receiving member or the print head in order to have different swaths correctly joining each other. An example of such a print process is an inkjet print process wherein an array of nozzles in a print head moves in a scanning direction to apply ink droplets as marking material by activating nozzles according to a digital signal comprising swath image data that is derived from a digital image. The term “print head” will be used for both a single print head comprising an array of print elements and for an assembly or plurality of single print heads that are fixed on a common mechanical structure, such as a carriage.
After making a swath over the full width of the image, or at least part of the full width, the image receiving member is advanced in a transport direction that is substantially perpendicular to the scanning direction in which the print head reciprocates. Equivalently the image receiving member may be fixed and the print head advanced. The advancement is stepwise and after making a paper step, the print process is applied to produce a next swath.
To have the various swaths joining each other contiguously, it is important that the paper step is accurately adjusted to the size of the swaths. If a paper step is too small, marking material, such as ink droplets, in image lines from two swaths will be printed on top of each other which may result in a dark line at the boundary between the two swaths. If a paper step is too large, a light line will appear at this boundary, because a part of the image receiving member will receive no marking material. To diminish the effects of a deviant paper step, a multi-pass strategy may be used wherein each individual paper step is a fraction of the swath width and wherein, in each swath, only part of the marking material that is needed to make up the image on the image receiving member, is applied. This is done at the expense of a diminished productivity. Other so-called print modes involve an interlacing print strategy in which image lines are printed alternated by non-printed lines. When the print head passes the same area again after a paper step, the non-printed lines are printed. This is useful when the integration density of a print head is smaller than the intended printed image line density on the image receiving member. In some print modes, the visibility of a dark or light line at the boundary may be more or less than in other, but in all cases it depends on the amount of overlap of the marking material in the two adjoining swaths.
A well-known method to diminish the visibility of these swath boundary lines is to interweave the swaths by modifying the shape of the swath boundary with a regular or irregular pattern. For this, a digital mask may recurrently be applied to pixels in the image lines at the boundary of a first swath. This digital mask prevents the activation of nozzles and thus the application of marking material in this first swath for at least a part of a number of these image lines from a print element close to an end of a print head up to the last print element at this end of the print head. A complementary digital mask is applied to the adjoining side of a second swath, masking the pixels that were printed in the first swath, thus complementing the image lines partly printed in the first swath. By this technique, the swath boundary is not parallel to the image lines in the swath and a small difference between the paper step and a predetermined distance within a swath will become less annoyingly visible. A small deliberate overlap of a number of image lines in a first swath and a second swath is all that is needed to apply this technique.
In a similar way, this technique may be applied for static staggered print heads extending in a direction perpendicular to the transport direction of the image receiving substrate. In that case, the substrate is transported with a constant speed and the swaths extend in a direction parallel to the transport direction. The swaths are printed simultaneously instead of in sequence. A small overlap between two neighbouring static print heads enables a variation of the boundary between the swaths by the application of complementary digital masks. Thus, even in absence of a paper step, the interweaving of swaths may be used to obfuscate a boundary line between them.
With swaths perpendicular to the medium transport direction, an advancement step or paper step is conventionally applied by a drive roller with encoders to control its rotation. The roller has a hard surface to exclude wear and minimize elasticity effects. Furthermore, the eccentricity of the roller is measured in a calibration and saved in an electronic memory for establishing a relation between the rotation of the roller and the lateral displacement of an image receiving member that is pressed against the surface of the roller. In practice, the actual distance over which the image receiving member is transported may deviate from the required paper step. This deviation is also known as the paper step error. It may result from an inaccurate calibration for a specific image receiving member, but more often the deviation results from changes in the image receiving member due to variation in humidity or temperature. These result in uncontrolled shrinkage or expansion of the image receiving member. A further source for a deviation may be the limited stiffness of the construction, the supporting frame, in which the image receiving member transport takes place as an acceleration or deceleration of the print head may slightly deform the structure in which an accurate fit of the two swaths is intended. The deviation may even not be constant over the whole width, especially when the image receiving member is wide as in the case of billboards, banners or engineering drawings.
In next generation print systems, the swath width and the associated paper step can be expected to increase substantially, because this increases the productivity of these systems. Using known image receiving member transport devices, this leads to an increasing deviation between the paper step or intended transport distance and the actual transport distance. Therefore, it is expected that the problem of an emerging light or dark boundary line at the transition between two swaths will increase. Measures to increase the accuracy are considered to be costly. Besides, there are parameters that are hard to control, such as the expansion of the image receiving member under humid conditions or its shrinkage when heated. Furthermore, it will be increasingly insufficient to apply a single paper step in a print system in the light of a paper step error that varies along the scanning direction.
Several measures are known that reduce the effect of a paper step error. In U.S. Pat. Nos. 5,384,587 and 6,547,370 the density of droplets in the image lines at the edges of a swath is reduced. The density is supplemented by droplets of an adjacent swath with the effect of spreading the deviation over a larger area on the image receiving member. Another approach is taken in U.S. Pat. No. 7,050,193, wherein the edge of a swath is measured to generate a signal to control the position of the print head when printing a next swath. This is similar to the method disclosed in U.S. Pat. No. 7,686,414 for printing on flexible substrates. In the latter case, image position control marks are detected to adapt the position of the image lines that are printed in a swath. This involves either a movement of the print head or a corrective data shift of the image data. In the second case, other print elements or nozzles than the originally intended ones are prepared to print corresponding image lines such that the image position moves relative to the image receiving member.
The known methods only have a limited effect on the reduction of the effects of a paper step error or involve far reaching measures, such as considerable data processing time. Hence, there is a need for an inexpensive, plain method that addresses the problems mentioned above.